1. Field of the Invention
The present invention relates to hydroprocessing methods and reactors and, more particularly, to a method of and apparatus for utilizing liquid quench to reduce pressure drop and increase throughput in a multiple bed hydrotreating-hydrocracking reactor.
2. Description of the Prior Art
The reaction of hydrocarbons, particularly heavier petroleum feedstocks such as distillates, lubricants, heavy oil fractions, residuum, etc., usually in the presence of a catalyst and elevated temperatures and pressures, is known as hydroprocessing. Typical hydroprocessing processes include hydrodesulfurization, hydrodenitrification, hydroisomerization, hydrodemetallation, hydrocracking, hydrogenation, and the like. For purposes of clarity herein, the term hydrotreating will be used to denote hydroprocessing reactions intended to remove contaminants from liquid hydrocarbon feedstocks, e.g., hydrodesulfurization, hydrodenitrification, hydrodemetallation, and the like.
Historically, hydrocracking catalysts have been particularly intolerant of contaminants, such as sulfur, nitrogen, metals and/or organometallic compounds, which are generally contained in heavy hydrocarbon liquid streams, particularly reduced crude oils, petroleum residua, tar sand bitumen, shale oil, liquified coal, reclaimed oil, and the like. These contaminants tend to deactivate catalyst particles during contact by the liquid hydrocarbon feed stream and hydrogen under hydroprocessing conditions. Therefore, it has become commonplace to catalytically hydrotreat the heavy liquid hydrocarbon feedstock to reduce to an acceptably low level its content of catalyst-poisoning contaminants before introducing the reduced contaminant feedstock to the first of the hydrocracking catalyst beds.
During the sequential processing of the liquid hydrocarbon feedstock, i.e., hydrotreating followed by hydrocracking, considerable heat is generated in each step. As a result, and in order to control the increase in temperature in the catalyst beds as the feedstock moves sequentially therethrough, it has become the practice to quench or cool the effluent reaction products from a prior catalyst bed before introducing them into the next catalyst bed. For this purpose a quench gas medium, such as recycle hydrogen gas, feed hydrogen gas or other suitable quench gases well known in the field, is injected into quench zones situated between the exit of one reaction zone and the entrance to the next zone. Generally, several beds with quench zones, typically from four to ten, are employed to control the increase in temperature in the beds. For example, a hydroprocessing reactor containing four reaction zones would likely have three quench gas injection points. In order to accomplish substantial hydrogen upgrading of the liquid feedstock, exothermal heat rise across each reaction zone will likely require that substantial quench gas be injected in quantities which may exceed the hydrogen gas being consumed by the hydroprocessing reaction occurring in the reactor. Although inter-bed quench gas introduction is effective as a means for reactor temperature profile control, the introduction of the additional quench gas increases the pressure drop across the reactor to a sufficient extent that it frequently limits the throughput capability of the reactor.
One proposed solution to the pressure drop problem is to reduce the quench gas flow as low as possible and operate with a maximum tolerable temperature rise between the feed inlet to the first reaction zone and the effluent from the final reaction zone. While this solution may be a viable compromise in some respects, inasmuch as catalyst fouling rate generally increases with increasing bed temperature, there is an economical price to be paid for this approach in terms of catalyst replacement rate.
As new hydrocracking catalysts have been developed which exhibit an improved tolerance to contaminants, particularly to metals, nitrogen and sulfur, contaminant-tolerant hydrocracking catalysts can replace the hydrotreating catalysts in the upper catalyst beds. This allows the liquid feedstock to be subjected to additional stages of hydrocracking in the same number of catalyst beds in the reactor with an attendant small increase in hydrocracking conversion to lighter products. However, the substitution of contaminant-tolerant catalysts for hydrotreating catalysts in some of the reactor beds does not contribute to a reduction in pressure drop or an increase in throughput if pressure drop is limiting throughput.
Currently a large number of hydrotreating-hydrocracking processes experience extremely high pressure drop and unfavorable throughput capability or undesirably high reactor temperature profiles and unfavorable catalyst life due to the consequences attending reactor temperature control utilizing inter-bed gas quenching. Accordingly, a multi-bed, multi-reaction zone hydrotreating-hydrocracking process that would permit satisfactory reactor temperature profile control while reducing pressure drop through the reactor and improving throughput through the catalyst beds would be desirable.